All over the world, the electric arc smelting furnace is considered to be an important apparatus in the production of steel from scrap and, increasingly, from pelletized iron sponge. This smelting apparatus is presently designed as a welded steel structure, protected against the high temperatures of the furnace by a refractory lining on the vessel wall as well as on the lid. The lining protects the steel structure against thermal, chemical and mechanical stresses, which are of sufficient magnitude to greatly limit the life of the refractory lining, e.g., 150 charges in the smelting area and about 100 charges at the lid. Accordingly, furnace production must be interrupted at intervals in order to install a new lining. These interruptions lead to a decrease in production and furthermore constitute a considerable percentage of operating expenses. For these reasons, water-cooled wall and lid treatments have been developed and water-cooled wall elements have been used to replace the refractory lining in portions of the furnace vessel outside of the smelting area. Wall elements of this type are already being offered for sale.
German Offenlegungsschrift No. 2 502 712 discloses water-cooled metal boxes to cool vessel walls of light arc furnaces. These boxes have the disadvantage that no reliable water force may be developed. Therefore, there is always a danger of water turbulence or dead flow areas in corners wherein water vapor and air bubbles may form. The formation of vapor and air bubbles causes a sudden drop in heat transfer causing local overheating which might lead to fusion of the box walls. Such burn-outs lead to additional operating disruptions and are a threat to the safety of furnace operation. Another disadvantage, being of importance in view of a possible energy recovery from the cooling water, is the fact that stress factors do not permit using cooling agents under increased pressure in connection with these cooling boxes. This would require thick-walled boxes necessitating an unduly heavy construction.
Another construction of cooling boxes for a blast furnace is disclosed in German Offenlegungsschrift No. 2 612 659. Each cooling box is provided with several cooling water channels bound by horizontal partitions whereby turbulence or vapor back-up is avoided. The cooling boxes are, however, imbedded in the furnace wall lining and are not pertinent to the teachings disclosed herein. Furthermore, the proposed cooling boxes comprise a very complex structure and the expensive casting process utilized to manufacture the cooling boxes results in a heavy construction.
A disclosed smelting furnace vessel (German Offenlegungsschrift No. 2 354 570) has a furnace wall constructed of coolers arranged at the furnace floor. Each cooler consists of a main body of cast iron or copper with a number of cooling pipes arranged within the main body. The coolers are provided either at the so-called hot spots of the furnace walls, or the walls themselves comprise several coolers. The furnace disclosed in German Offenlegungsschrift No. 2 354 570 has many disadvantages. The furnace wall of cooling blocks is relatively heavy necessitating a reinforced construction for the bottom portion of the furnace. Due to the thickness of the ingot and poor heat transfer between the casting and cooling pipes, the cooling blocks are subject to considerable wear by combustion requiring replacement after a certain period of operation. Since relatively thin-walled cooling pipes must be utilized in order to permit sufficient heat dissipation, operation with a cooling medium under high pressure is not possible. Due to the great mass of the cooling blocks, the temperature of the cooling medium is relatively stable vis-a-vis a change in thermal stress caused by actual furnace conditions. Thus, this system is not entirely suitable to control a furnace operation.
It is the object of the present invention to provide a vessel for a metal smelting furnace, particularly an electric furnace, of the type mentioned above, which is of a simple, relatively light, structure and relatively inexpensive to manufacture. Another object of the invention is to provide a cooling system operable under high pressure resulting in an improved heat transfer at the vessel walls. Simultaneously, the vessel wall has a low heat change inertia in order to facilitate control of the furnace operation. Moreover, the furnace wall is constructed whereby a minimum of combustion of wall material occurs. The invention solves the above-discussed shortcomings of the prior art by providing a vessel interior wall which includes at least one cooling pipe coil, spaced from the vessel center, and comprising thick-walled pipes or pipe sections arranged in a closely adjacent relation. The result is a furnace vessel of straightforward design, lightweight construction and which may be erected at reduced manufacturing costs. The light construction of the furnace wall requires no reinforced structure for the bottom part of the furnace. The furnace wall construction permits faultless forced guidance of the cooling water thus largely avoiding thermal stress and local overheating which results in a prolongation of the life of the furnace. The pipes used for the cooling coils have a great torsion and bending strength as well as a high resistance--due to the wall thickness--to shock-like external mechanical stress. Residual thermal stress is better absorbed by a circular pipe cross-section than, for example, by a rectangular cross-section.
In accordance with another feature, the cooling pipe coil consists of U-shaped or helically coiled pipes, whereby the adjacent sections of the cooling pipe coil are arranged either parallel with or at right angle to the furnace vessel axis. From a technological point of view, it has been found advantageous to manufacture the cooling pipe coils by bending. The pipe coils may be interconnected by welding. According to additional features of the invention, the cooling pipe coil may be constructed so that its pipes and/or pipe sections are longitudinally partitioned and connected by a plate towards the center of the vessel.
The cooling pipe coil may comprise pipe sections open on both ends with the open ends of two adjacent pipe sections being interconnected by welded caps. In accordance with another advantageous feature of the invention, the open pipe ends at each side of the coil are seal-welded to a compact joint with deflection channels arranged opposite the pipe ends. According to another feature of the invention, the cooling pipe coil is connected at one end to the cooling medium supply line and at the other end to the cooling medium discharge line, whereby the cooling medium supply line and discharge line is formed by at least one downwardly extending pipe. In order to enlarge the interior pipe surface, to improve thermal conductivity, the inside of the pipe may be profiled.
During furnace operation, a slag layer of about 10 to 20 mm thickness forms on the pipe surfaces facing the center of the vessel. This slag layer acts as a thermal and electric insulator decreasing the risk of local overheating even further, reducing thermal stress and achieving a decrease in energy dissipation via the cooling water. The electric insulating effect of the slag layer offers extensive protection against disruptive discharge flashover and/or secondary electric arcs from the scrap to the cooling coil. Such disruptive discharges, however, are improbable, as the electric resistance over the great distance across the pipe wall is too high. In order to further the formation of a slag layer on the pipe surface, the cooling pipe coil is, at the side close to the furnace center, equipped with prominences, preferably burls, pins, ribs, or the like.
It has been found extremely advantageous to have cooling fluid under high pressure flow through the cooling pipe coil. The use of forced flow for the cooling water ensures greater safety of operation and avoids turbulence or stationary flow zones with the attendant risk of local overheating by formation of vapor pockets. Moreover, forced flow provides uniform cooling and lower thermal stress and relatively exact adjustment of the flow velocity as well as flow volume. The cooling pipe coil is best suited for the forced flow of cooling water.
According to additional features of the invention, the individual cooling pipe coils are easily replaced in case of damage by dismantling them in either the radial or axial direction. For safety reasons, another feature of the invention provides that the lowest portion of each cooling coil is welded to the inside rim of a circular drain running essentially at right angle to the furnace interior wall to trap water. A trench may be provided at the furnace shell below the lower edge of the drain.
In one embodiment of the furnace vessel of the present invention, the wall is made up solely of the cooling pipe coils which are attached to girders forming a wall skeleton. Portions of this skeleton may be arranged to be tiltable or detachable doors each consisting of a girder frame encompassing one or several cooling pipe coils. In a further favorable development of the invention it is suitable to match the cooling pipe coils to the shape of the vessel--seen in planview--by bending, folding or juxtaposing. Each cooling pipe coil surface may be provided with an insulating coating of, for example, a ceramic material, such as enamel.
The furnace wall construction of the invention makes it possible to use a fast-reacting monitor system. The ratio between heat dissipated by the cooling water and heat supplied by electric energy is particularly favorable in the operation of an electric smelting furnace with a vessel built in accordance with the invention. The favorable thermal conditions as well as the longer life of the vessel wall are further promoted by conditions existing during the melting down phase of the smelting cycle. For example, the cooling effect of the charge material and protection against light arc radiation due to the large effective capacity of the furnace. Moreover, the slag layer on the cooling coil surface facilitates heat transfer between the furnace chamber, pipe wall, and cooling water, while the respective slag cover on the smelt reduces heat radiation off the bath.